This is Pavonis Mons (Click to enlarge, you’ll be glad you did).
This is what once was a active volcanic mountain that has since become inactive. Originally discovered by Mariner 9 in 1971, it was recorded and not considered very noteworthy up until recently. Now it is being highly considered as a potential landing sight for human travel to this planet. The former volcano could potentially shield Mars colonists from the harsh conditions of the planet.
It is unknown how deep the cave system could go, but it is estimated that it could be anywhere from 50m (164ft) all the way to 250m (820ft)! This could potentially harbor life due to heating below the ground. This is an additional benefit to picking such a feature as a potential human landing site.
Additional photos of the cave